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Neutron polarizability: some numbers



The neutron electric polarizability was derived in 1988-90 from
measurements of the transmission of neutrons through lead as a
function of their wavelength. The experiments were done by
Schmiedmayer et al. at Harwell (England) and Oak Ridge.

The result is a = (0.9 +/- 0.5)x10^-3 fm^3.

What does that mean? If a polarizable object is placed in an
electric field, an electric dipole moment will be induced.
d = (4 pi epsilon_o) a E.

We can write
(4 pi epsilon_o) = 1.1x10^-10 F/m = 7x10^-7 e V^-1 fm^-1,
where e is the electron charge.

Thus, for a = 10^-3 fm^3,
d = 7x10^-10 e-fm / (V/fm)

That is, an electric field of 1 V/fm (roughly 1V across the diameter
of the neutron) would produce a dipole moment equivalent to the
separation of +e and -e charges by a distance of 7x10^-10 fm. Not
too large!

Norman Ramsey sent me a copy of a paper "Particle Physics with Cold
Neutrons" by D. Dubbers that appeared in Progress in Particle and
Nuclear Physics Vol 26, p 173 (1990) that contained these results.

By the way, the electric polarizability of the proton is similar in
magnitude (1.07 +/- 0.11)x10^-3 fm^-3, which one would suspect, since
they're both made of three quarks. Theoretical references are in
Nuclear Instruments and Methods, A284, 137 (1989).

Paul Zitzewitz, Professor of Physics
Department of Natural Sciences
University of Michigan-Dearborn
4901 Evergreen Rd
Dearborn, MI 48128
(313) 593-5158 FAX (313) 593-4937